The present invention pertains to precipitation hardening austenitic superalloys for use in neutron radiation environments. It especially relates to gamma prime hardening austenitic superalloys.
In fast breeder nuclear reactors of the liquid metal type, as well as in other, the fuel is encapsulated in cladding, typically of cylindrical form. A capsule containing the fuel is usually referred to as a fuel element or fuel rod. In accordance with the teachings of the prior art, the cladding is composed of stainless steel, typically AISI 316 stainless steel. The ducts through which the liquid metal (typically sodium) flows are also composed of this 316 steel. In practice, difficulty has been experienced both with the cladding and the ducts. The stainless steel on being bombarded by neutrons, particularly where the neutron flux is epithermal (E&gt;0.1 MeV), swells. In addition, the stainless steel does not have the necessary strength at the elevated temperatures, 500.degree. C. and higher, at which the reactors of the type involved operate. The problem is particularly serious in the case of the cladding. On being heated by the fission reaction, the fuel in tne capsules expands and in addition gas is generated and exerts high pressure at the high temperatures within the capsules. The cladding is highly stressed. The stress exerted in the ducts is at a lower level both because the temperature of the ducts is lower than that of the cladding and also because the mechanical pressure to which the ducts are subjected is lower. The stainless steel of the cladding and of the ducts is subject to substantial creep which is accentuated by the neutron irradiation.
Various alloys have been considered in efforts to provide improved cladding and duct materials. Among the alloys studied have been gamma prime hardened austenitic superalloys such as those described in U.S. Pat. Nos., 3,199,978; 4,129,462; 4,172,742; 4,231,795; and 4,236,943. In addition to the metallurgical conditions described in the listed patents, some of these alloys have also been studied in a solution treated and cold worked condition, as described in copending application Ser. No. 248,121 filed on Mar. 27, 1981 which issued as U.S. Pat. No. 4,359,350 on Nov. 16, 1982. These gamma prime austenitics can generally be designed to have good swelling resistance, high strength and high stress rupture strength relative to austenitic alloy 316.
The post irradiation ductility of these alloys as a class has been found to be at, or near, zero depending on alloy composition, heat treatment, irradiation temperature and fluence. Exposing gamma prime austenitics to a high energy neutron (E&gt;0.1 MeV) flux in the as cold worked condition, as described in U.S. Pat. No. 4,359,350, has provided some improvement in the post irradiation ductility. However, the post irradiation ductility still is a concern, and there exists a need for further improvement in this area.